The unrest delayed by more than five hours the start of the WTO meeting. Opening ceremonies were cancelled because delegates' motorcades were unable to reach the venue after protesters chained themselves together and lay down at road junctions.

Plans for United Nations Secretary-General Kofi Annan and US Secretary of State Madeleine Albright to address an opening session were scrapped.

Instead the WTO went straight into the first of a series of plenary sessions, where trade ministers are allowed to address the meetings.

Protesters called on police to stop firing tear gas

Deep division among the world's trading blocs was already in evidence on the first day.

The European Union, with the support of Japan tabled a paper proposing a wide agenda for the trade talks, including issues like investment and competition policy, which the US opposes.

BBC economics correspondent Andrew Walker says there is also a gulf between the members on how far Europe and Japan should go in exposing their farmers to more foreign competition.

Trade talk targets

Expand tariff cuts to agriculture and services

Set agenda for other areas of trade liberalisation

Discuss labour rights

Set standards for 'fair' trading

And many developing countries think proposed talks on labour standards are an attempt to protect rich country industry from low-wage competition.

For their own part, the poor countries want to reconsider their earlier commitments to open their markets to imports. They also want the rich countries to allow in more textiles and clothing imports.

Meanwhile the Cairns group of farm exporting nations, with the support of the United States, was reportedly preparing a plan to eliminate all agricultural export subsidies - something the EU, which spends $40bn each year on agricultural support, is bound to object to.

The protesters represent a wide spectrum of causes - from concerns about the impact of global trade on labour rights and the environment, to the influence of international organisations. They say the free trade ideals espoused by the WTO benefit big business at the cost of workers, the environment and communities.